A gunman opened fire inside an Orlando nightclub early Sunday morning, killing 50 people and wounding at least 53. The nightclub, Pulse, located in Orlando's downtown area, is one of the city's largest gay nightclubs, according to The New York Times.

"This is an incident, as I see it, that we certainly classify as domestic terror incident," said Orange County Sheriff Jerry Demings.

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The assailant, whom officials identified as 29-year-old Omar Mateen from Fort Pierce, Florida, used an assault rifle, handgun and possibly an explosive device, the Times reported. He was killed in a shootout with police.

When asked during a news conference if Mateen had a connection to radical Islamic terrorism, FBI agent Ron Hopper said they "have suggestions that individual has leanings towards that." Mateen was on authorities' radar for at least five years and was characterized a "known quantity" to law enforcement, CBS News reported. Before he started shooting, the gunman called 9-1-1 and pledged his allegiance to ISIS. He also mentioned the Boston bombers during the call.

Orlando Police officers direct family members away from a fatal shooting at Pulse Orlando nightclub in Orlando, Florida.

The response has been overwhelming. The line at the One Blood donation center in Orlando was snaked around the block Sunday morning. (Note: Earlier reports indicated the ban on gay and bisexual men donating had been lifted temporarily. One Blood clarified that these reports were false in a tweet stating FDA regulations were still in effect and urged eligible donors to give.)

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The incident began around 2 a.m. inside the club and spilled outside, where an officer attempted to confront the shooter. The gunman then went back inside, continued shooting and took hostages. Around 5 a.m., nine police officers raided the club in a rescue attempt, detonating two explosive devices as a distraction. A shootout ensued. The gunman was killed and a police officer was wounded.

An Orange County Sheriff's Department SWAT member arrives to the scene of a fatal shooting at Pulse Orlando nightclub.

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Police cars surround the Pulse Orlando nightclub.

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The firearms used by Mateen were purchased in Florida within the past week, according to Orlando Police. He was able to purchase them with limited background checks because he worked at a security firm since 2007, ABC reports.

The Washington Post described a chilling text message conversation between Mina Justice and her 30-year-old son Eddie, who was inside the club. Eddie, the Post said, texted his mom when the shooting began and told her to call police. He ran into the bathroom with other patrons to hide, later texting his mom: "He's coming."

Justice said the next message from her son was: "He has us, and he's in here with us." That was the last conversation she had with her son.

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Someone else inside the club described a scene in which a group of patrons tore down a wall to escape the shooter.

Pulse posted a message on its Facebook page around 2 a.m., saying: "Everyone get out of pulse and keep running." Thousands of comments have poured on to the club's Facebook page this morning, with people posting updates and others messages of hope and love.

In a statement issued Sunday morning, GLAAD said it stands in solidarity with the LGBT community in Orlando.

Our hearts break for the victims and families of this horrific act of violence. We stand in solidarity with the LGBTQ community in #Orlando.

NBC News obtained a statement from the gunman's father, who said his son was angered a couple of months ago when he saw two men kissing in Miami. "We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," he said, adding, "This has nothing to do with religion."

New York Times correspondent Rukmini Callimachi noted that while ISIS and al-Qaeda have not claimed credit, the attack comes during the Islamic holy month of Ramadan, and that ISIS spokesman Abu Muhammad al-Adnani urged "Lone Wolves" to hit soft targets in the U.S. and Europe.

14. Adnani encourages Lone Wolves to therefore hit any & all targets in Europe & America that will engender civilian deaths during Ramadan